Organ formation in plants occurs via the activity of apical meristems. Plant meristems contain a pool of stem cells, which are able to self-maintain, and give rise to a variety of cell types including cells required for organ initiation. The initiation and maintenance of stem cells and their integration into organ-forming meristems are thus the basis for continuous plant development.
The Wuschel protein, designated hereafter as WUS, plays a key role in the initiation and maintenance of the apical meristem, which contains a pool of pluripotent stem cells (Endrizzi et al., 1996, Plant Journal 10:967-979; Laux et al., 1996, Development 122:87-96; and Mayer et al., 1998, Cell 95:805-815). Arabidopsis plants mutant for the WUS gene contain stem cells that are misspecified and that appear to undergo differentiation. WUS encodes a novel homeodomain protein, which presumably functions as a transcriptional regulator (Mayer et al., 1998, Cell 95:805-815). The stem cell population of Arabidopsis shoot meristems is believed to be maintained by a regulatory loop between the CLAVATA (CLV) genes which promote organ initiation and the WUS gene which is required for stem cell identity, with the CLV genes repressing WUS at the transcript level, and WUS expression being sufficient to induce meristem cell identity and the expression of the stem cell marker CLV3 (Brand et al. (2000) Science 289:617-619; Schoof et al. (2000) Cell 100:635-644). Constitutive expression of WUS in Arabidopsis has been recently shown to lead to adventitious shoot proliferation from leaves (in planta) (Laux, T., Talk Presented at the XVI International Botanical Congress Meeting, Aug. 1-7, 1999, St. Louis, Mo.).
There is a great deal of interest in identifying the genes that encode proteins involved in development in plants, generally toward the objective of altering plant growth and architecture. WUS represents one such gene. However, the WUS gene can also be used for the novel application of stimulating in vitro growth of plant tissue and improving transformation. In this manner, this gene can expand the range of tissues types targeted for transformation. Specifically, the WUS gene may be used to improve meristem transformation frequencies and could result in genotype independent transformation of many important crops such as maize, soybean and sunflower. Furthermore, transformation into meristems would stimulate the formation of new apical initials reducing the chimeric nature of the transgenic events. Lastly, ectopic expression into non-meristematic cells would stimulate adventive meristem formation. This could lead to transformation of non-traditional tissues such as leaves, leaf bases, stem tissue, etc. Alternatively, transformation of a more traditional target such as callus or the scutellum of immature embryos could promote a “non-traditional” growth response, i.e. meristems in place of somatic embryos. In addition, WUS may also be used as a genetic marker for meristems.
Modulation of WUS is expected to modulate plant and/or plant tissue phenotype including cell growth stimulation, organogenesis, and somatic embryogenesis. WUS may also be used to improve transformation via somatic embryogenesis. Expression of Arabidopsis WUS can induce stem cells in vegetative tissues, which can differentiate into somatic embryos (Zuo, et al. (2002) Plant J 30:349-359). The ability to stimulate organogenesis and/or somatic embryogenesis may be used to generate an apomictic plant. Apomixis has economic potential because it can cause any genotype, regardless of how heterozygous, to breed true. It is a reproductive process that bypasses female meiosis and syngamy to produce embryos genetically identical to the maternal parent. With apomictic reproduction, progeny of specially adaptive or hybrid genotypes would maintain their genetic fidelity throughout repeated life cycles. In addition to fixing hybrid vigor, apomixis can make possible commercial hybrid production in crops where efficient male sterility or fertility restoration systems for producing hybrids are not available. Apomixis can make hybrid development more efficient. It also simplifies hybrid production and increases genetic diversity in plant species with good male sterility.
Accordingly, the availability of nucleic acid sequences encoding all or a portion of a WUS protein would facilitate studies to better understand programmed development in plants, provide genetic tools to enhance the efficiency of gene transfer into meristem tissue and help provide alternative transformation methods in several important crops.